1. Field of the Invention
The present invention relates generally to a fluid-filled resilient bushing, and more particularly to a technique for providing the bushing with excellent damping characteristic for axial vibrations, based on a flow resistance of a fluid and inertia of the fluid.
2. Discussion of the Prior Art
A vibration damping support for a vibration system in an automotive vehicle is known as a cylindrical resilient bushing wherein a resilient member is interposed between an inner sleeve and an outer sleeve. One type of such a resilient bushing is adapted primarily for damping and/or isolating vibrations applied thereto in its axial direction. For instance, this type of resilient bushing is utilized as a cross-member mount, a body mount, a tension-rod bushing, a strut mount, and other mounts for resiliently mounting or connecting elements of a vehicle which are subject to vibrations.
The vibration damping mounts or bushings (hereinafter referred to as "resilient bushing") indicated above are generally required to exhibit high damping characteristics for a vibrational load applied thereto in the axial direction. To this end, the conventional resilient bushings employ a rubber block or resilient member which has excellent damping properties. Upon application of an axial vibrational load, the resilient member undergoes elastic deformation, thereby damping the vibration.
In the conventional resilient bushing wherein the resilient member has high damping characteristic due to its elastic deformation, NVH properties (Noise, Vibration and Harshness) of the bushing tend to be deteriorated, since the high damping characteristic of the resilient member necessarily results in a high dynamic spring constant or rate of the resilient member. To avoid excessively high dynamic spring constant, the bushing cannot be provided with sufficiently high damping characteristic.
On the other hand, fluid-filled resilient bushings are proposed as disclosed in U.S. Pat. Nos. 3,642,268 and 3,698,703. These fluid-filled bushings are capable of effectively damping input vibrations based on a flow resistance of a fluid and inertia of the fluid through the bushing, without increasing the dynamic spring constant of the resilient member. However, conventional fluid-filled bushings as disclosed in the above documents are designed to receive vibrations in the radial direction, primarily for damping the radial vibrations. Therefore, these fluid-filled resilient bushings cannot be used for effectively damping vibrations applied in the axial direction.